Fix to uninstall shoes and test our manifests

We generate two non-Ruby files in the Gem bin dir--`shoes` (during gem
install) and `shoes-backend` (during the first run or picking a shoes
backend).

The `shoes` binary wasn't getting uninstalled because we weren't putting the
`lib/rubygems_plugin.rb` file into the gem itself. That plugin is what's
responsible at gem uninstallation time for removing shoes since rubygems
itself doesn't know about the `shoes` executable.

`shoes-backend` was a recently added file that got missed on adding to
cleanup when I created it.

We've had at least two problems resulting from files in the repo that
didn't get added into a manifest and hence were missing from the gem.
Now there's a test that will confirm that every .rb file in the repo
shows up in at least one of the manifests.

Time to start testing for it!
I feel like this could be `reject`, e.g. 

```ruby
incorrectly_excluded_files = Dir["**/*.rb"].reject do |file|
  all_manifest_files.include?(file)
end
```

should do roughly the same and get rid off the ternary and the compact, or am I missing something?